diff --git a/CHANGELOG.md b/CHANGELOG.md
index 28f7da47..059972a8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,9 @@ See [Conventional Commits](https://conventionalcommits.org) for commit guideline
 
 ### Fixed
 
+* Preserve round trips for discriminated map codecs with strict output schemas,
+  including values used as another codec's output, without narrowing permissive
+  intermediate output policies.
 * Accept contextual Dart member names and deeply snapshot JSON-owned generated
   fields without changing opaque codec values. Preserve effective nullable
   overrides when inferring explicit JSON boundaries.
diff --git a/packages/ack/CHANGELOG.md b/packages/ack/CHANGELOG.md
index fe301be1..73345f92 100644
--- a/packages/ack/CHANGELOG.md
+++ b/packages/ack/CHANGELOG.md
@@ -2,6 +2,9 @@
 
 ### Fixed
 
+* Accept union-owned discriminators in strict map-backed codec outputs while
+  preserving declared discriminator rules and rejecting unrelated extra keys.
+  Already-permissive intermediate outputs retain their policies and values.
 * Expose the shared `deepUnmodifiableCopy` helper for generated immutable
   constructors. It snapshots nested collections without validating data or
   changing opaque noncollection objects.
diff --git a/packages/ack/lib/src/schemas/codec_schema.dart b/packages/ack/lib/src/schemas/codec_schema.dart
index 4a83b31d..a9b80ab2 100644
--- a/packages/ack/lib/src/schemas/codec_schema.dart
+++ b/packages/ack/lib/src/schemas/codec_schema.dart
@@ -185,10 +185,16 @@ final class CodecSchema<Boundary extends Object, Runtime extends Object>
   }
 
   @override
-  CodecSchema<Boundary, Runtime> copyWithInner(AnyAckSchema newInner) {
+  CodecSchema<Boundary, Runtime> copyWithInner(AnyAckSchema newInner) =>
+      _copyWithSchemas(input: newInner as AckSchema<Boundary, Object>);
+
+  CodecSchema<Boundary, Runtime> _copyWithSchemas({
+    AckSchema<Boundary, Object>? input,
+    AckSchema<dynamic, Runtime>? output,
+  }) {
     return CodecSchema._(
-      inputSchema: newInner as AckSchema<Boundary, Object>,
-      outputSchema: outputSchema,
+      inputSchema: input ?? inputSchema,
+      outputSchema: output ?? outputSchema,
       decoder: _decoder,
       encoder: _encoder,
       decoderIdentity: _decoderIdentity,
diff --git a/packages/ack/lib/src/schemas/discriminated_object_schema.dart b/packages/ack/lib/src/schemas/discriminated_object_schema.dart
index 91cb0aa7..0973455e 100644
--- a/packages/ack/lib/src/schemas/discriminated_object_schema.dart
+++ b/packages/ack/lib/src/schemas/discriminated_object_schema.dart
@@ -246,6 +246,8 @@ final class DiscriminatedObjectSchema<T extends Object>
   /// The effective schema includes this union's discriminator property as an
   /// exact branch literal, even when the authored branch omitted it. Wrappers
   /// around the branch (codecs, defaults) are preserved.
+  /// Object-backed codec outputs that omit the key accept its optional literal,
+  /// so decoded maps remain valid after union normalization adds the key.
   ///
   /// When [optionalDiscriminator] is true the injected literal is optional.
   /// This is used only on the encode path, where a branch encoder may legally
@@ -265,15 +267,76 @@ final class DiscriminatedObjectSchema<T extends Object>
       );
     }
 
-    return effectiveDiscriminatedBranch(
-          discriminatorKey: discriminatorKey,
-          discriminatorValue: discriminatorValue,
-          branchSchema: branchSchema,
-          optionalDiscriminator: optionalDiscriminator,
-        )
+    final effective = effectiveDiscriminatedBranch(
+      discriminatorKey: discriminatorKey,
+      discriminatorValue: discriminatorValue,
+      branchSchema: branchSchema,
+      optionalDiscriminator: optionalDiscriminator,
+    );
+    return _withRuntimeDiscriminator(effective, discriminatorValue)
         as AckSchema<JsonMap, T>;
   }
 
+  AnyAckSchema _withRuntimeDiscriminator(
+    AnyAckSchema schema,
+    String discriminatorValue,
+  ) {
+    if (schema is ObjectSchema) {
+      if (schema.additionalProperties ||
+          schema.properties.containsKey(discriminatorKey)) {
+        return schema;
+      }
+      // A decoder may omit the union-owned key; normalization adds it later.
+      // Its output must also accept that normalized value on subsequent use.
+      return effectiveDiscriminatedObjectBranch(
+        discriminatorKey: discriminatorKey,
+        discriminatorValue: discriminatorValue,
+        objectSchema: schema,
+        optionalDiscriminator: true,
+      );
+    }
+    if (schema is CodecSchema) {
+      final input = _withRuntimeDiscriminator(
+        schema.inputSchema,
+        discriminatorValue,
+      );
+      final output = _withRuntimeDiscriminator(
+        schema.outputSchema as AnyAckSchema,
+        discriminatorValue,
+      );
+      return identical(input, schema.inputSchema) &&
+              identical(output, schema.outputSchema)
+          ? schema
+          : schema._copyWithSchemas(input: input, output: output);
+    }
+    if (schema is AnyOfSchema) {
+      final branches = [
+        for (final branch in schema.schemas)
+          _withRuntimeDiscriminator(branch, discriminatorValue),
+      ];
+      if (const ListEquality<AnyAckSchema>(
+        IdentityEquality(),
+      ).equals(branches, schema.schemas)) {
+        return schema;
+      }
+      return AnyOfSchema(
+        List.unmodifiable(branches),
+        isNullable: schema.isNullable,
+        isOptional: schema.isOptional,
+        description: schema.description,
+        constraints: schema.constraints,
+        refinements: schema.refinements,
+      );
+    }
+    if (schema is WrapperSchema) {
+      final inner = _withRuntimeDiscriminator(schema.inner, discriminatorValue);
+      return identical(inner, schema.inner)
+          ? schema
+          : schema.copyWithInner(inner);
+    }
+    return schema;
+  }
+
   @override
   @protected
   SchemaResult<T> parseWithContext(Object? value, SchemaContext context) {
diff --git a/packages/ack/test/schemas/discriminated_runtime_validation_test.dart b/packages/ack/test/schemas/discriminated_runtime_validation_test.dart
index 6a3b3a36..040bc2a8 100644
--- a/packages/ack/test/schemas/discriminated_runtime_validation_test.dart
+++ b/packages/ack/test/schemas/discriminated_runtime_validation_test.dart
@@ -171,6 +171,264 @@ void main() {
     });
   });
 
+  group('map codec outputs with union-owned discriminators', () {
+    for (final wrapped in [false, true]) {
+      for (final emitsDiscriminator in [false, true]) {
+        test('round-trips strict ${wrapped ? 'wrapped' : 'plain'} outputs '
+            '${emitsDiscriminator ? 'with' : 'without'} a discriminator', () {
+          final output = Ack.object({'name': Ack.string()});
+          final branch = Ack.object({'name': Ack.string()}).codec<JsonMap>(
+            decode: (map) => {
+              if (emitsDiscriminator) 'kind': 'cat',
+              'name': map['name'],
+            },
+            encode: (map) => {'name': map['name']},
+            output: wrapped ? output.withDefault({'name': 'Mittens'}) : output,
+          );
+          final union = Ack.discriminated<JsonMap>(
+            discriminatorKey: 'kind',
+            schemas: {
+              'cat': wrapped ? branch.withDefault({'name': 'Mittens'}) : branch,
+            },
+          );
+          final wire = <String, Object?>{'kind': 'cat', 'name': 'Mittens'};
+          final parsed = union.parse(wire)!;
+          expect(parsed, wire);
+          expect(() => parsed['kind'] = 'dog', throwsUnsupportedError);
+          expect(union.encode(parsed), wire);
+          final codec = _codecWithOutput(union, parsed);
+          expect(codec.parse('wire'), same(parsed));
+          expect(codec.encode(parsed), 'wire');
+          // Adapting a branch must not widen the original output schema.
+          expect(
+            _codecWithOutput(output, wire).safeParse('wire').isFail,
+            isTrue,
+          );
+        });
+      }
+    }
+
+    test('round-trips through chained map codecs', () {
+      final inner = Ack.object({'name': Ack.string()}).codec<JsonMap>(
+        decode: (map) => {'name': map['name']},
+        encode: (map) => {'name': map['name']},
+        output: Ack.object({'name': Ack.string()}),
+      );
+      final union = Ack.discriminated<JsonMap>(
+        discriminatorKey: 'kind',
+        schemas: {
+          'cat': inner.codec<JsonMap>(
+            decode: (map) => map,
+            encode: (map) => map,
+            output: Ack.object({'name': Ack.string()}),
+          ),
+        },
+      );
+      final wire = <String, Object?>{'kind': 'cat', 'name': 'Mittens'};
+      expect(union.encode(union.parse(wire)!), wire);
+    });
+
+    for (final unionOutput in [false, true]) {
+      test('preserves permissive intermediate codec outputs '
+          '${unionOutput ? 'inside anyOf' : 'directly'}', () {
+        final output = Ack.object({
+          'name': Ack.string(),
+        }).passthrough().refine((map) => map['kind'] == 'internal');
+        final inner = Ack.object({'name': Ack.string()}).codec<Object>(
+          decode: (map) => {'name': map['name'], 'kind': 'internal'},
+          encode: (value) => {'name': (value as JsonMap)['name']},
+          output: unionOutput ? Ack.anyOf([output]) : output,
+        );
+        final union = Ack.discriminated<String>(
+          discriminatorKey: 'kind',
+          schemas: {
+            'cat': inner.codec<String>(
+              decode: (value) => (value as JsonMap)['name']! as String,
+              encode: (name) => {'name': name, 'kind': 'internal'},
+            ),
+          },
+        );
+        final wire = <String, Object?>{'kind': 'cat', 'name': 'Mittens'};
+        expect(union.parse(wire), 'Mittens');
+        expect(union.encode('Mittens'), wire);
+        final codec = _codecWithOutput(union, 'Mittens');
+        expect(codec.parse('wire'), 'Mittens');
+        expect(codec.encode('Mittens'), 'wire');
+      });
+    }
+
+    test('still rejects conflicts in permissive final map outputs', () {
+      final union = Ack.discriminated<JsonMap>(
+        discriminatorKey: 'kind',
+        schemas: {
+          'cat': Ack.object({'name': Ack.string()}).codec<JsonMap>(
+            decode: (map) => {'name': map['name'], 'kind': 'internal'},
+            encode: (map) => {'name': map['name']},
+            output: Ack.object({'name': Ack.string()}).passthrough(),
+          ),
+        },
+      );
+      expect(
+        union.safeParse({'kind': 'cat', 'name': 'Mittens'}).isFail,
+        isTrue,
+      );
+      expect(
+        union.safeEncode({'kind': 'internal', 'name': 'Mittens'}).isFail,
+        isTrue,
+      );
+    });
+
+    test('adapts a nested codec output without running its conversions', () {
+      final output = Ack.object({'name': Ack.string()}).codec<JsonMap>(
+        decode: (_) => throw StateError('Output decoder must not run'),
+        encode: (_) => throw StateError('Output encoder must not run'),
+        output: Ack.object({'name': Ack.string()}),
+      );
+      final union = Ack.discriminated<JsonMap>(
+        discriminatorKey: 'kind',
+        schemas: {
+          'cat': Ack.object({'name': Ack.string()}).codec<JsonMap>(
+            decode: (map) => {'name': map['name']},
+            encode: (map) => {'name': map['name']},
+            output: output,
+          ),
+        },
+      );
+      final wire = <String, Object?>{'kind': 'cat', 'name': 'Mittens'};
+      final parsed = union.parse(wire)!;
+      expect(union.encode(parsed), wire);
+      final codec = _codecWithOutput(union, parsed);
+      expect(codec.parse('wire'), same(parsed));
+      expect(codec.encode(parsed), 'wire');
+    });
+
+    test('round-trips strict object outputs inside a union', () {
+      final union = Ack.discriminated<Object>(
+        discriminatorKey: 'kind',
+        schemas: {
+          'cat': Ack.object({'name': Ack.string()}).codec<Object>(
+            decode: (map) => {'name': map['name']},
+            encode: (value) => value as JsonMap,
+            output: Ack.anyOf([
+              Ack.object({'name': Ack.string()}),
+              Ack.instance<int>(),
+            ]),
+          ),
+        },
+      );
+      final wire = <String, Object?>{'kind': 'cat', 'name': 'Mittens'};
+      final parsed = union.parse(wire)!;
+      expect(union.encode(parsed), wire);
+      final codec = _codecWithOutput(union, parsed);
+      expect(codec.parse('wire'), same(parsed));
+      expect(codec.encode(parsed), 'wire');
+      expect(codec.safeEncode({...wire, 'extra': true}).isFail, isTrue);
+    });
+
+    final union = Ack.discriminated<JsonMap>(
+      discriminatorKey: 'kind',
+      schemas: {
+        'cat': Ack.object({'name': Ack.string()}).codec<JsonMap>(
+          decode: (map) => {'name': map['name']},
+          encode: (map) => {'name': map['name']},
+          output: Ack.object({
+            'name': Ack.string(),
+            'details': Ack.object({'age': Ack.integer()}).optional(),
+          }),
+        ),
+      },
+    );
+    final invalidCases = <({String name, JsonMap value, String path})>[
+      (name: 'missing name', value: {'kind': 'cat'}, path: '#/name'),
+      (
+        name: 'wrong name type',
+        value: {'kind': 'cat', 'name': 42},
+        path: '#/name',
+      ),
+      (
+        name: 'unrelated extra property',
+        value: {'kind': 'cat', 'name': 'Mittens', 'extra': true},
+        path: '#/extra',
+      ),
+      (
+        name: 'nested discriminator property',
+        value: {
+          'kind': 'cat',
+          'name': 'Mittens',
+          'details': {'age': 1, 'kind': 'cat'},
+        },
+        path: '#/details/kind',
+      ),
+    ];
+    for (final testCase in invalidCases) {
+      test('still rejects ${testCase.name}', () {
+        final codec = _codecWithOutput(union, testCase.value);
+        for (final result in [
+          codec.safeParse('wire'),
+          codec.safeEncode(testCase.value),
+          union.safeEncode(testCase.value),
+        ]) {
+          expect(result.isFail, isTrue);
+          expect(_errorPaths(result.getError()), contains(testCase.path));
+        }
+      });
+    }
+
+    test('preserves an explicitly authored output discriminator', () {
+      final union = Ack.discriminated<JsonMap>(
+        discriminatorKey: 'kind',
+        schemas: {
+          'cat': Ack.object({'name': Ack.string()}).codec<JsonMap>(
+            decode: (map) => {'kind': 'dog', 'name': map['name']},
+            encode: (map) => {'name': map['name']},
+            output: Ack.object({
+              'kind': Ack.literal('dog'),
+              'name': Ack.string(),
+            }),
+          ),
+        },
+      );
+      final runtime = <String, Object?>{'kind': 'cat', 'name': 'Mittens'};
+      final codec = _codecWithOutput(union, runtime);
+      expect(codec.safeParse('wire').isFail, isTrue);
+      expect(codec.safeEncode(runtime).isFail, isTrue);
+      expect(union.safeEncode(runtime).isFail, isTrue);
+    });
+
+    test('membership never reruns branch conversions or refinements', () {
+      var decodes = 0;
+      var encodes = 0;
+      var refinements = 0;
+      final union = Ack.discriminated<JsonMap>(
+        discriminatorKey: 'kind',
+        schemas: {
+          'cat': Ack.object({'name': Ack.string()}).codec<JsonMap>(
+            decode: (map) {
+              decodes++;
+              return {'name': map['name']};
+            },
+            encode: (map) {
+              encodes++;
+              return {'name': map['name']};
+            },
+            output: Ack.object({'name': Ack.string()}).refine((_) {
+              refinements++;
+              return true;
+            }),
+          ),
+        },
+      );
+      final runtime = <String, Object?>{'kind': 'cat', 'name': 'Mittens'};
+      final codec = _codecWithOutput(union, runtime);
+      expect(codec.parse('wire'), same(runtime));
+      expect(refinements, 1);
+      expect(codec.encode(runtime), 'wire');
+      expect(refinements, 2);
+      expect(decodes, 0);
+      expect(encodes, 0);
+    });
+  });
+
   group('direct discriminated encoding callback counts', () {
     test('map branches do not acquire an extra runtime refinement pass', () {
       var refinements = 0;